Centrifugal pumps are well known devices employed to pump fluids from one location to another. Many pumps today are called upon to operate on critical projects over a wide range of capacities without any manual intervention. For these applications, the pumps must be able to operate continuously without interruption.
A common problem with centrifugal pumps is that they do a very poor job of pumping gas or multiphase fluids and can easily become air-locked or “vapor-locked” causing them to deliver reduced performance and ultimately lose prime without warning. Centrifugal pumps can become air bound from many sources such as vortexing from improper suction line submergence, leaks on the suction side of the pump, entrained air in the pumping fluid, cavitation due to poor suction conditions and from suction recirculation. Some of these problems can be controlled to some extent by various means but all of them cannot be completely eliminated.
Suction recirculation is one of the major causes of air-locking in centrifugal pumps and is very difficult to prevent. Suction recirculation is a phenomenon that occurs in all centrifugal pumps when operated at off-peak performance. The capacity at which suction recirculation occurs is directly related to the design suction specific speed of the pump. The higher the suction specific speed, the closer will be the beginning of recirculation to the capacity at best efficiency.
Suction recirculation is the reversal of flow at the impeller eye. At operation away from best efficiency, a portion of the flow is redirected out of the impeller eye (instead of through the impeller exit vane tips) in a swirling motion due to the mismatch between the incoming fluid flow and the rotation of the impeller inlet blades. The swirling fluid travels out of the impeller eye upstream of the impeller inlet into the suction piping causing a distortion of the fluid pressure field. In the fluid pressure field, the heavier fluid is thrown outward by the centripetal action of the rotating impeller blades while the lighter vapor (air) is centrifuged toward the center of rotation. This creates a vapor bubble or blockage directly in the eye of the impeller preventing any new liquid from passing through the eye to the discharge side of the impeller. At this point, the pump will stop pumping (lose prime) and is said to be air-locked. In addition to not being able to perform its required task of pumping the fluid, air-locking also causes excessive noise and vibration and can damage the internals of the pump. Damaging the internals of the centrifugal pump could include pitting the impeller, wear ring of casing, rupturing the mechanical seals, excessively loading the bearings or bending or breaking the pump shaft.
The problem with air locking is very common for pumps that are used on varying capacity applications with back pressure such as sewer bypass projects where the pump's pumping capacity can often exceed the incoming fluid rates (called “snore” condition) during non-peak hours and the pump is required to pump through a pressurized forcemain. When this occurs, the fluid level in the sump drops below the minimum submergence level of the suction entrance allowing air to enter the suction piping. The air in the suction piping reduces the overall flow rate into the pump causing the pump to undergo suction recirculation. Once the suction recirculation cycle begins, the pump is no longer able to develop enough centrifugal head to penetrate the forcemain and the pump becomes air bound.